This invention relates to a new and improved evaporative heat exchanger for use with evaporative air conditioning units. More particularly, the invention relates to a new and improved plate for fabrication of plate-type evaporative heat exchangers.
In the past, evaporative heat exchangers were fabricated from thick, metal plates. Channels were formed by the placement of spacers between these plates. The thickness of the plates provided rigid support for the plates as high velocity air was blown through the channels during operating conditions. Metal plates were expensive because cost was based upon the weight of the metal. Placement of the spacers betwen the plates resulted in added material cost as well as labor costs for assembly.
When water was sprayed, dripped or injected into the channels where evaporation occurred, water would not adequately cover the surfaces of the plates. Only a portion of the surface of the plates was covered with water. Since effective cooling through evaporation can occur only where water contacts the surface of the plates, cooling of the plates was limited to the wetted surface which, in turn, resulted in poor operating performance.
In certain evaporative heat exchangers, the plates were fabricated from materials such as plastic that exhibited hydrophobic characteristics. Thus, water tended to bead on the surface and, when sufficiently accumulated, the water would flow out of the channels where evaporation occurred. Beading of water indicates resistance of the water to disperse and wet the surface of the plates. Instead of a thin film of water distributed over the entire surface of the plate, beads of water occupied only portions on the surface of the plate. As a result, unwetted portions of the plate cannot contribute to evaporative cooling. Further, the depth of the water bead acts as an insulator, thereby, inhibiting heat transfer.
Large evaporative air conditioning units for small-scale commercial buildings such as schools, light manufacturing facilities and the like must deliver volumes of conditioned air in the range of twenty five thousand cubic feet per minute (25,000 cfm) at a temperature of fifty five degrees Fahrenheit (55.degree. F.). Two major design considerations for large evaporative air conditioning units are the aesthetics of the housing and cost competitiveness with heat pumps and Freon air conditioners.
Architects who design small-scale commercial buidings prefer that large evaporative air conditioning units for rooftop installation have an appearance of a short, box-shaped housings. Any other housing configurations, particularly tall housings, or added components to the box-shaped housing are undesirable because these features detract from the aesthetics of the building.
Shipping a large evaporative air conditioning unit can be expensive if the unit is exceptionally wide. The 1987 regulations promulgated by Interstate Commercial Commission require that trucks carrying a load wider than welve feet (12') must pay extra fees for an overwidth permit and must have an escort vehicle accompanying this truck in route.
One manufacturer manufactures an evaporative air conditioning unit having an aluminum plate-type heat exchanger. The plate comprises two thin, flat sheets of aluminum separated by and attached to a corrugated sheet of aluminum to form a series of parallel channels between the corrugations for conditioned air. The outer surfaces of this plate are laminated with a sheet of material capable of being wetted. A series of these plates are attached together by spacers to form channels for evaporative cooling. This heat exchanger operates nominally at 3,600 cubic feet of conditioned air per minute at 55.degree. F. Calculations indicate that approximately seven (7) of these heat exchangers would be required to deliver 25,000 cubic feet per minute at 55.degree. F. Seven (7) juxtaposed heat exchangers of this type would result in a large evaporative air conditioning unit with its housing being approximately fourteen feet (14') wide.
Another manufacturer also manufactures an evaporative air conditioning unit having an aluminum plate-type heat exchanger. Each plate is formed with a set of truncated dimples that protrude from the surface of the plate. These dimples serve a spacing function to separate the plates without spacers and to define channels therebetween when assembled into an evaporative heat exchanger. Furthermore, the truncated dimples interupt the boundary layer of air flowing through the air conditioning channels to enhance the heat transfer capability of the heat exchanger. An epoxy resin coats the aluminum plates within the channels where evaporation occurs to prevent corrosion of such plates. However, this coating is inherently hydrophobic. This heat exchanger operates nominally at 17,000 cubic feet per minute at 55.degree. F. To achieve performance of 25,000 cubic feet of conditioned air per miute at 55.degree. F., the housing for this evaporative air conditioning unit incorporating such a heat exchanger would be approximately thirteen and a half feet (13.5') wide.
It is from these considerations and others that the present invention evolved.